Ants, even fire ants, are not a problem. They are most likely not eating the grapes because fire ants only go for high protein food like meat. In any case the ants are eating something and leaving behind ant poop and ant carcasses. The ant remains will be decomposed by some kind of microbes that may or may not be in your pile already. In any case it will all happen in good time. You will also see cockroaches, flies, mosquitoes, pill bugs, grubs, fruit flies, earwigs, worms, lizards, and all sorts of wildlife in your pile. Don't be alarmed. They are all in there decomposing something.
Your pile of wood chips is a pile of very stable carbohydrates called cellulose and lignin. The fungi that decompose wood can be found growing on fallen logs in your nearby wooded area. Possibly the best thing you could do for your pile is to bring one of those logs into your pile and make a nest for it on top. Then keep the pile moist. I have tried many ways of keeping my pile wet. I even rigged up fine spray shower heads over top of my pile. Even that fine spray was too much water and wasted a lot without wetting the entire pile. What I have finally decided is about right is the little misting nozzles you can sometimes find at hardware stores. Mine are not the misting nozzles for drip irrigators but the misting nozzles that put out fog to cool the air when it is really hot and dry out (click here to see one brand like I'm talking about
). These misters really work just like I had hoped. They wet the pile from the outside and the moisture just soaks in. You have to leave them on a long time (like a week at a time), but they hardly use any water at all. Wood chips take forever to decompose so don't get in too much of a hurry. Keeping them moist is the single best thing you can do (after getting the rotten log in there). Also I would not bury the log. Wood decomposes from the outside in, not the inside out. The log and your pile needs plenty of air.
Regarding horse manure: get as much as you can and bury it in the pile. Fresh manure stinks (SURPRISE!!) but when you bury it under the wood chips or dry leaves, all that smell is absorbed by the wood/leaves. That smell is the smell of both carbohydrates and ammonia (nitrogen) leaving the pile so capturing it in the wood is exactly what you want to do.